Thermoplastic resins are used for various processing or molding methods, such as calendering, injection molding and extrusion, for producing extensive and wide variety of shaped products because of their excellent moldability and colorability. Ordinarily, depending on a species and a usage, a thermoplastic resin may be blended with various additives, such as thermal stabilizer, antioxidant, flame-retardant, ultraviolet absorber, lubricant, plasticizer, filler, pigment, antistatic agent, impact modifier, processability modifier, and foaming agent, to form a thermoplastic resin composition, which is used for processing or molding. During the processing or molding, the thus-blended additive can be separated and precipitated to be deposited on surfaces of metallic parts of processing or molding machines, such as a calender roller, an embossing roller, a mold and an extrusion die. This phenomenon is called “plateout”, and the deposited material is also called “plateout”. In addition to the precipitation of such an additive, an impurity or a low-molecular weight component in the thermoplastic resin can also be precipitated and deposited to result in “plateout” in some cases. Further, “plateout” can be caused by a combination of both of these, or a combination with another factor, in some cases. Thus, in this description, a “phenomenon” of deposition of a component of a thermoplastic resin composition on a metallic part surface of a processing or molding machine, and also a “deposited material”, are inclusively called “plateout”.
Plateout, when caused, results in inferior products, such as a surface-roughened molded product or a molded product with inferior size accuracy. Further, it becomes necessary to terminate the operation and effect cleaning for removal of the plateout, which results in a lowering of productivity and requires a troublesome work. In many cases, plateout is obviated by selecting species and adjusting amounts of components, or adjusting the processing conditions, such as a processing temperature, etc., but the countermeasure has to be selected by trial and error because the measure can be different depending on a cause of the plateout.
In the field of processing of vinyl chloride resin, powdery silicon dioxide is known to function as an anti-plateout agent because of its large surface area effective for scraping out and absorbing a potential plateout substance, as described in “Handbook of Polyvinyl Chloride Formulating”, Edited by Edward J. Wickson, John Wiley & Sons (1993), and “Plastics Additives and Modifiers Handbook”, Edited by Jesse Edenbaum, Van Nostrand Reinhold (1993). While depending on the grade of silicon dioxide powder, an amount of 0.3–2% with respect to polyvinyl chloride compound is described to be effective for preventing plateout.
However, the addition in an amount as large as about 1 wt. % of silicon dioxide powder to a polyvinyl chloride compound adversely affects the impact resistance of the compound and results in a lowering in transmittance undesirably for use as a transparent product.
Further, commercially available silicon dioxide powder has a very low bulk density and is inferior in powder handling characteristic. More specifically, in the case of pneumatically transporting a resin composition containing silicon dioxide powder added to a thermoplastic resin, the fine powdery silicon dioxide is liable to go out of the system because of difficulty in capture thereof by a fine powder recovery apparatus, such as a cyclone or a bag filter. This is not only uneconomical but also undesirable as a possible source of environmental pollution.
Japanese Patent Publication (JP-B) 5-61302 has disclosed a method of providing a synthetic resin powder having improved flowability and anti-blocking property by adding fine powder of oxide of a metal, such as Si, to synthetic resin powder obtained by coagulation of a graft copolymer latex. However, the publication lacks any reference to plateout, and according to our review, such a fine powder of oxide of Si, etc., added by powder blending can only show a very low effect of plateout prevention.
Japanese Laid-Open Patent Application (JP-A) 8-81605 has disclosed to prevent plateout by adding 0.1–1 wt. part of calcium carbonate fine powder of 0.1–0.5 μm to 100 wt. parts of vinyl chloride resin. The publication describes that calcium carbonate fine powder of below 0.1 μm results in an inferior anti-plateout effect. The publication contains Examples 1 and 2 directed to compositions containing 0.1 wt. part and 1 wt. part, respectively, of calcium carbonate fine powder, and Example 2 exhibited a sufficient anti-plateout effect, whereas Example 1 exhibited a lower anti-plateout effect which is considered to be insufficient. This may be construed to be attributable to the lower addition amount of calcium carbonate fine powder. According to out knowledge, such calcium carbonate fine powder as used in the publication suffers from conspicuous dusting and is not desirable from the viewpoint of operation environment, while the publication does not refer to it. Further, in view of its Examples, the publication appears to envisage an opaque product, and a lowering in transparency appears to be inevitable when applied to a transparent product.
WO 96/34036 has disclosed an agglomerate of polymer fine particles obtained by adding a colloidal silica aqueous dispersion to an aqueous polymer latex and agglomerating the latex to provide such an agglomerate of polymer fine particles having captured therein primary particles of colloidal silica, and also a molded product having improved rigidity, toughness and thermal resistance due to the colloidal silica dispersed at a high concentration formed by molding the agglomerate. The publication describes that the colloidal silica is preferably added in an amount of 1–500 wt. parts per 100 wt. parts of the polymer and is added in a range of 25 to 150 wt. parts per 100 wt. parts of the polymer. Thus, the publication does not refer to any about a processability, particularly an anti-plateout effect, of a thermoplastic resin composition containing a lower concentration of colloidal silica as may be obtained by further blending the above-obtained agglomerate of polymer fine particles having captured colloidal silica particles with another thermoplastic resin.